The present invention relates to a perch channel reception device and a perch channel reception method of a mobile radio communication terminal which executes radio communication according to CDMA (Code Division Multiple Access).
Description of the Prior Art
Mobile radio communication terminals such as portable cellular phones have become widely available these days. Generally, a mobile radio communication terminal has to search for base stations around the mobile radio communication terminal and obtain base station information (information with regard to a base station, such as present time of the base station, an ID of the base station, transmission power of the base station, etc.) which is transmitted by each of the base stations, when power of the mobile radio communication terminal is turned on or at predetermined time periods.
The base station information which is transmitted by a base station so as to be received by mobile radio .communication terminals in its own base station cell (area) is included in a channel which is called xe2x80x9cperch channelxe2x80x9d. Therefore, the mobile radio communication terminal has to receive and demodulate the perch channel of each of the base stations first, in order to obtain the base station information of the base stations.
In a mobile radio communication terminal which executes radio communication according to CDMA (Code Division Multiple Access), information transmitted by a desired base station can be demodulated from multiplexed data (that is, a received radio wave which has been modulated according to CDMA and transmitted by base stations) by taking the correlation between the received modulated radio wave and a predetermined scrambling code of the base station in sync with predetermined scrambling timing of the base station.
The modulated radio wave which is received by the mobile radio communication terminal includes frame boundary symbols of the base stations around the mobile radio communication terminal, in which each frame boundary symbol indicates information concerning the scrambling timing of a base station and the type of the scrambling code used by the base station. The mobile radio communication terminal can detect the scrambling timing and the scrambling code of the perch channel of each base station, by detecting the frame boundary symbols included in the received modulated radio wave.
After the perch channel reception of the base stations around the mobile radio communication terminal is completed, the mobile radio communication terminal starts next procedure for establishing communication with the most suitable one of the base station. After the communication between the mobile radio communication terminal and the most suitable base station is started, the mobile radio communication terminal periodically executes the perch channel reception of base stations around the mobile radio communication terminal, while executing the communication with the most suitable base station.
FIG. 1 is a block diagram showing a conventional perch channel reception device of a mobile radio communication terminal according to CDMA, for executing the perch channel reception with respect to base stations around the mobile radio communication terminal.
The perch channel reception device shown in FIG. 1 includes a delay profile generator 1, a base station scrambling timing detector 2, a correlator with time window 4, and a perch channel demodulator 5.
The delay profile generator 1 detects frame boundary symbols which are included in a received modulated radio wave, and thereby generates a delay profile of base stations around the mobile radio communication terminal. There are a plurality of base stations around the mobile radio communication terminal, and thus the delay profile generated by the delay profile generator 1 includes a plurality of peaks corresponding to the base stations around the mobile radio communication terminal. The base station scrambling timing detector 2 detects scrambling timing of each of the base stations based on the delay profile which has been generated by the delay profile generator 1 and thereby generates a base station scrambling timing table which indicates the detected scrambling timing of each of the base stations around the mobile radio communication terminal. Thereafter, the base station scrambling timing detector 2 selects a base station from the base stations one by one, and informs the timing corrector 3 about the scrambling timing of the selected base station (i.e. base station scrambling timing). The base station scrambling timing detector 2 also determines the type of the scrambling code used by the selected base station based on the frame boundary symbol corresponding to the selected base station, and informs the correlator with time window 4A about the type of the scrambling code used by the selected base station (i.e. the type of base station scrambling code). The correlator with time window 4, which is provided with a time window of a fixed size, takes the correlation between (compares) the received modulated radio wave and the base station scrambling code (i.e. the scrambling code of the selected base station which has been detected by the base station scrambling timing detector 2), with respect to a plurality of comparison starting times (that is, the time when the comparison between the received modulated radio wave and the base station scrambling code is started) in the fixed time window around the base station scrambling timing, and thereby generates correlation data. The perch channel demodulator 5 demodulates the correlation data which has been generated by the correlator with time window 4 into logic data which indicates the base station information of the selected base station such as present time of the selected base station, an ID of the selected base station, transmission power of the selected base station, etc.
As described above, in the conventional perch channel reception device of a mobile radio communication terminal according to CDMA, the correlator with time window 4 generates correlation data between the received modulated radio wave and the base station scrambling code, with respect to a plurality of comparison starting times in the fixed time window around the base station scrambling timing.
Generally, each base station is provided with a high precision oscillator, while precision of the oscillator of each mobile radio communication terminal is not high enough. The oscillation frequency of the oscillator of a mobile radio communication terminal has an error of the order of PPM due to variations in the environment in which the mobile radio communication terminal is used, limited size and price of the mobile radio communication terminal, etc. Further, in the conventional perch channel reception device of FIG. 1, there is a large time difference between the generation of the delay profile and the completion of the perch channel reception of the base stations around the mobile radio communication terminal. The perch channel reception becomes impossible if the scrambling timing used by the mobile radio communication terminal is shifted by 1 chip (chip: a time period corresponding to one scrambled code when logic data is scrambled) in comparison with real scrambling timing of the selected base station. Therefore, the conventional perch channel reception device has to be provided with a time window of a very large size in order to absorb and compensate the shift of the scrambling timing during the perch channel reception of all the base stations.
In the following, the shift of the scrambling timing between the base station and the mobile radio communication terminal will be explained in detail referring to FIG. 2. Incidentally, the following explanation will be given with respect to a case where the number of base stations around the mobile radio communication terminal is three, the scrambling frequency (chip frequency) is 4 MHz, and reception time necessary for the perch channel reception of one base station is 50 ms, for the sake of simplicity. Referring to FIG. 2, the scrambling code proceeds 200000 chips in the 50 ms reception time for perch channel reception of a base station. Thereafter perch channel reception of the next base station is started. If we assume the oscillation frequency error of the oscillator of the mobile radio communication terminal is +3 PPM, the scrambling timing shift of the mobile radio communication terminal after the 50 ms perch channel reception becomes 0.6 chips, as shown in FIG. 2. In the same way, the scrambling timing shift becomes as large as 1.2 chips at the point when perch channel reception of the third base station is started. In CDMA, perch channel reception (signal demodulation) generally becomes impossible if the scrambling timing shift grows 1 chip, therefore, the correlator with time window 4 has to be provided with the time window in order to absorb and compensate the scrambling timing shift. Generally, the number of base stations around a mobile radio communication terminal is 5xcx9c20, therefore, if we assume the scrambling frequency (chip frequency) is 4 MHz and the perch channel reception time for one base station is 50 ms and the oscillator frequency error of the mobile radio communication terminal is xc2x13 PPM, the correlator with time window 4 has to be provided with a time window that can cover +12 chips.
The correlator with time window 4 is usually implemented by a plurality of correlators, and generally, the number of the correlators is proportional to the size of the time window of the correlator with time window 4. Therefore, in the conventional perch channel reception device, the correlator with time window 4 has to be provided with a large number of correlators in order to absorb and compensate the large scrambling timing shift, and thus circuit scale of the correlator with time window 4 is necessitated to be considerably large. On the other hand, if we limit the circuit scale of the correlator with time window 4, frequency precision of the oscillator of the mobile radio communication terminal is required to be raised, and thus manufacturing cost and the price of the mobile radio communication terminal is necessitated to be high.
It is therefore the primary object of the present invention to provide a perch channel reception device of a mobile radio communication terminal according to CDMA, by which the shift of the scrambling timing can be absorbed and compensated by means of small circuit composition and thereby the perch channel reception can be executed successfully, even if a considerable amount of difference occurred between scrambling frequencies (oscillator frequencies) of the mobile radio communication terminal and the base station.
Another object of the present invention is to provide a perch channel reception method of a mobile radio communication terminal according to CDMA, by which the shift of the scrambling timing can be absorbed and compensated by means of small circuit composition and thereby the perch channel reception can be executed successfully, even if a considerable amount of difference occurred between scrambling frequencies of the mobile radio communication terminal and the base station.
Another object of the present invention is to provide a computer-readable record medium storing a program for instructing a computer or a DSP (digital signal processor) of a mobile radio communication terminal according to CDMA to execute processes for successively receiving perch channels transmitted by base stations around the mobile radio communication terminal, by which the shift of the scrambling timing can be absorbed and compensated by means of small circuit composition and thereby the perch channel reception can be executed successfully, even if a considerable amount of difference occurred between scrambling frequencies of the mobile radio communication terminal and the base station.
In accordance with a first aspect of the present invention, there is provided a perch channel reception device of a mobile radio communication terminal according to CDMA for successively receiving perch channels transmitted by base stations around the mobile radio communication terminal. In the perch channel reception device, on each completion of perch channel reception of a base station, the time difference between real scrambling timing of the currently received base station and scrambling timing which has been used for executing perch channel reception of the currently received base station is detected, and a timing correction is calculated as the accumulation of the time difference, and thereby scrambling timing of the next base station which has been detected based on a delay profile is corrected using the timing correction. Perch channel reception of the next base station is executed using the corrected scrambling timing of the next base station.
In accordance with a second aspect of the present invention, in the first aspect, the time difference between the real scrambling timing of the currently received base station and the scrambling timing which has been used for executing perch channel reception of the currently received base station is detected by a correlator means with time window. The correlator means with time window generates correlation data with respect to the currently received base station by taking the correlation between a received modulated radio wave and scrambling code of the currently received base station with respect to a plurality of comparison starting times in a time window around the corrected scrambling timing which is used for the perch channel reception of the currently received base station.
In accordance with a third aspect of the present invention, in the second aspect, the perch channel reception device comprises a timing corrector means for calculating the corrected scrambling timing of the next base station using the scrambling timing of the next base station which has been detected based on the delay profile and the accumulation of the time difference detected by the correlator means with time window, and supplying the corrected scrambling timing of the next base station to the correlator means with time window so as to be used for the perch channel reception of the next base station.
In accordance with a fourth aspect of the present invention, in the first aspect, the perch channel reception device comprises a delay profile generator means, a base station scrambling timing detector means, a base station scrambling code detector means, a sequencer means, a timing corrector means, a correlator means with time window, and a perch channel demodulator means. The delay profile generator means detects frame boundary symbols which are included in a received modulated radio wave and thereby generates the delay profile of base stations around the mobile radio communication terminal. The base station scrambling timing detector means detects scrambling timing of each of the base stations based on the delay profile which has been generated by the delay profile generator means. The base station scrambling code detector means detects the type of scrambling code of each of the base stations based on the frame boundary symbols which have been detected by the delay profile generator means. The sequencer means selects a base station from the base stations one by one as the object of perch channel reception. The timing corrector means calculates corrected scrambling timing of the selected base station using the scrambling timing of the selected base station which has been detected by the base station scrambling timing detector means and the timing correction. The correlator means with time window takes the correlation between the received modulated radio wave and the scrambling code of the selected base station which has been detected by the base station scrambling code detector means, with respect to a plurality of comparison starting times in a time window around the corrected scrambling timing of the selected base station, and thereby generates correlation data with respect to the selected base station. When perch channel reception of the selected base station is completed, the correlator means with time window detects a time difference between real scrambling timing and the corrected scrambling timing of the selected base station, and thereby calculates a time difference between the real scrambling timing of the selected base station and the scrambling timing of the selected base station which has been detected by the base station scrambling timing detector means, and supplies the time difference to the timing corrector means as the timing correction. The perch channel demodulator means demodulates the correlation data generated by the correlator means with time window into logic data including information concerning the selected base station.
In accordance with a fifth aspect of the present invention, in the fourth aspect, the correlator means with time window includes three or more correlators, a correlation intensity comparator means, an output selector means, and an accumulator means. Different comparison starting times in the time window around the corrected scrambling timing of the selected base station are set to the three or more correlators. Each correlator takes the correlation between the received modulated radio wave and the scrambling code of the selected base station in sync with its comparison starting time, and thereby outputs correlation data and correlation intensity. The correlation intensity comparator means executes comparison between the correlation intensity supplied from the correlators, selects one correlator having the largest correlation intensity, and outputs a selection signal which designates the selected correlator. The correlation intensity comparator means also outputs timing shift data which indicates the time difference between real scrambling timing and the corrected scrambling timing of the selected base station. The output selector means, which is supplied with the correlation data from the correlators, selects correlation data that is supplied from the correlator designated by the selection signal supplied from the correlation intensity comparator means, and outputs the selected correlation data to the perch channel demodulator means. The accumulator means accumulates the timing shift data supplied from the correlation intensity comparator means, and supplies the accumulated timing shift to the timing corrector means as the timing correction.
In accordance with a sixth aspect of the present invention, in the fifth aspect, each of the correlators included in the correlator means with time window is implemented by a correlator of correlator bank type.
In accordance with a seventh aspect of the present invention, in the fifth aspect, each of the correlators included in the correlator means with time window is implemented by a correlator of matched filter type.
In accordance with an eighth aspect of the present invention, in the fourth aspect, the timing corrector means is implemented by an adder for adding the timing correction supplied from the correlator means with time window to the scrambling timing of the selected base station which has been detected by the base station scrambling timing detector means.
In accordance with a ninth aspect of the present invention, there is provided a perch channel reception method of a mobile radio communication terminal according to CDMA for successively receiving perch channels transmitted by base stations around the mobile radio communication terminal. In the perch channel reception method, on each completion of perch channel reception of a base station, the time difference between real scrambling timing of the currently received base station and scrambling timing which has been used for executing perch channel reception of the currently received base station is detected, and a timing correction is calculated as the accumulation of the time difference, and thereby scrambling timing of the next base station which has been detected based on a delay profile is corrected using the timing correction. Perch channel reception of the next base station is executed using the corrected scrambling timing of the next base station.
In accordance with a tenth aspect of the present invention, in the ninth aspect, the perch channel reception method comprises a delay profile generation step, a base station scrambling timing detection step, a base station scrambling code detection step, a selection step, a timing correction step, a perch channel reception step, a timing correction calculation step, and a perch channel demodulation step. In the delay profile generation step, frame boundary symbols which are included in a received modulated radio wave is detected and thereby the delay profile of base stations around the mobile radio communication terminal is generated. In the base station scrambling timing detection step, scrambling timing of each of the base stations is detected based on the delay profile which has been generated in the delay profile generation step. In the base station scrambling code detection step, the type of scrambling code of each of the base stations is detected based on the frame boundary symbols which have been detected in the delay profile generation step. In the selection step, a base station is selected from the base stations one by one as the object of perch channel reception. In the timing correction step, corrected scrambling timing of the selected base station is calculated using the scrambling timing of the selected base station which has been detected in the base station scrambling timing detection step and the timing correction. In the perch channel reception step, the correlation between the received modulated radio wave and the scrambling code of the selected base station which has been detected in the base station scrambling code detection step is taken with respect to a plurality of comparison starting times in a time window around the corrected scrambling timing of the selected base station, and thereby correlation data with respect to the selected base station is generated. In the timing correction calculation step, a time difference between real scrambling timing and the corrected scrambling timing of the selected base station is detected as a result of the perch channel reception step and thereby the timing correction, which indicates the time difference between the real scrambling timing of the selected base station and the scrambling timing of the selected base station which has been detected in the base station scrambling timing detection step, is calculated when the perch channel reception step with respect to the selected base station is completed. In the perch channel demodulation step, the correlation data generated in the perch channel reception step is demodulated into logic data including information concerning the selected base station.
In accordance with an eleventh aspect of the present invention, there is provided a computer-readable record medium storing a program for instructing a computer or a DSP (digital signal processor) of a mobile radio communication terminal according to CDMA to execute processes for successively receiving perch channels transmitted by base stations around the mobile radio communication terminal. In the processes, on each completion of perch channel reception of a base station, the time difference between real scrambling timing of the currently received base station and scrambling timing which has been used for executing perch channel reception of the currently received base station is detected, and a timing correction is calculated as the accumulation of the time difference, and thereby scrambling timing of the next base station which has been detected based on a delay profile is corrected using the timing correction. Perch channel reception of the next base station is executed using the corrected scrambling timing of the next base station.
In accordance with a twelfth aspect of the present invention, in the eleventh aspect, the processes include a delay profile generation step, a base station scrambling timing detection step, a base station scrambling code detection step, a selection step, a timing correction step, a perch channel reception step, a timing correction calculation step, and a perch channel demodulation step. In the delay profile generation step, frame boundary symbols which are included in a received modulated radio wave is detected and thereby the delay profile of base stations around the mobile radio communication terminal is generated. In the base station scrambling timing detection step, scrambling timing of each of the base stations is detected based on the delay profile which has been generated in the delay profile generation step. In the base station scrambling code detection step, the type of scrambling code of each of the base stations is detected based on the frame boundary symbols which have been detected in the delay profile generation step. In the selection step, a base station is selected from the base stations one by one as the object of perch channel reception. In the timing correction step, corrected scrambling timing of the selected base station is calculated using the scrambling timing of the selected base station which has been detected in the base station scrambling timing detection step and the timing correction. In the perch channel reception step, the correlation between the received modulated radio wave and the scrambling code of the selected base station which has been detected in the base station scrambling code detection step is taken with respect to a plurality of comparison starting times in a time window around the corrected scrambling timing of the selected base station, and thereby correlation data with respect to the selected base station is generated. In the timing correction calculation step, a time difference between real scrambling timing and the corrected scrambling timing of the selected base station is detected as a result of the perch channel reception step and thereby the timing correction, which indicates the time difference between the real scrambling timing of the selected base station and the scrambling timing of the selected base station which has been detected in the base station scrambling timing detection step, is calculated when the perch channel reception step with respect to the selected base station is completed. In the perch channel demodulation step, the correlation data generated in the perch channel reception step is demodulated into logic data including information concerning the selected base station.